The present invention is directed to a device for automatically measuring both the near and far visual acuity of a patient's eyes in a simple and objective manner.
At the present time, there are basically two method for measuring the visual acuity of the eyes. One of these methods utilizes information provided by the person being tested (subjective testing) and the other method uses external measuring devices (objective testing). In the subjective method, vision testers such as charts, etc. have been widely used for many years. However, these vision testers provide no accurate measurement because they rely on the personal evaluation of the patient in comparing the images of a target while various corrective lenses are placed in front of the eyes.
Subjective tests are particularly unsuitable for measuring the visual acuity of children's eyes. For example, this year in the United States, over 21/2 million six year old children will enter more than 60,000 elementary schools. The American Association for the Prevention of Blindness estimates that one out of every twenty preschool children has a vision problem which, if uncorrected, will interfere with the child's development and education. Young children are unable to subjectively determine whether their vision is "good"or "bad". These young children cannot respond to conventional chart tests because they cannot read. Special devices such as the Landolt C and Pointing E are used to test the visual acuity of children. Since all these tests are subjective, special training and high motivation are necessary in order to obtain accurate measurements. Consequently, subjective testing methods are difficult to administer, unreliable, time-consuming, and totally impractical for mass vision testing of preschool children.
Objective testing methods are much more effective in measuring the visual acuity of the eyes, particularly the eyes of children. Although automatic objective measuring devices are now available on the market, these devices are not practical for mass vision testing. Existing devices must be operated by trained specialists, usually optometrists or ophthalmologists and they generally are large pedestal mounted devices which occupy a great amount of space and are not portable. Also, since these existing objective devices are complex mechanisms with considerable electronics, the price of these devices is prohibitively high.
As a result of these disadvantages of the existing objective devices, visual acuity is often determined through the use of Snellen letter chart in which the threshold letter size for the subject is found and converted to a visual acuity measurement. This procedure is subject to several subtle variables which can significantly affect the outcome. For example, inappropriate room lighting, test lamp aging, failure of the examinee to cover each eye properly, examiner recording error and inherent testing pressures all affect the visual acuity measurement. Furthermore, the results of such a visual acuity measurement again are often seriously effected by the desires of the patient. Thus, it is often difficult to ascertain the visual acuity of the patient using such charts and other similar subjective devices.
A number of objective devices have been proposed which overcome some of the disadvantages of the subjective devices but which do not overcome all the previously mentioned disadvantages of the objective devices. In U.S. Pat. No. 3,824,005 issued to Westman on July 16, 1974, a refractometer is shown in which the eye observes a target image and the reflected light from the retina of the patient's eye is transformed into an electrical signal that reaches a peak when the target image on the retina is in focus. The apparatus includes a primary bar or target which is illuminated by a light source and observed by the patient's eye through various lenses. The target image formed on the retina is reflected via a beam splitter to a photodetector which is connected to a circuit arrangement to generate an electrical signal related to the visual acuity of the eye. A secondary bar pattern, which is identical to the primary target pattern, is positioned between the beam splitter and the photodetector and vibrated by a vibrator to periodically block and unblock the reflected target image from the retina of the patient's eye. When the patient's eye is in focus on the primary target pattern and the secondary bar pattern is not in a blocking position, the intensity of the light detected by the photodetector is at a maximum. In addition, the primary target pattern is constantly moved toward and away from the patient's eye to permit the image of the primary target pattern to be periodically in focus on the retina of the eye regardless of its refracted state. As a result, the measurement of the position of the axially moving primary target at the instant the amplitude of the light transmitted through the secondary bar pattern is at its maximum will provide an indication of the refractive state of the patient's eye.
Another prior art system is shown in U.S. Pat. No. 3,888,569 issued to Munnerly et al. on June 10, 1975 in which a refractometer is shown having an adjustable compensating lens system for focusing the target image on the retina of the eye. The primary target pattern in the Munnerly patient is movable and the secondary pattern positioned adjacent the photodetector is fixed. The signal received by the photodetector is a function of the focus of the image of the primary target pattern on the retina of the eye. A sharply focused bar pattern produces a less intense signal. The visual acuity of the patient's eye is calculated by a computer which controls the focus and position of the patterns and selects and stores results from the signal detected by the photodetector. A compensating lens system is also provided in the Munnerly patent which is controlled by the digital computer and used to adjust the focus of the patient's eye.
Several prior art patents show refractometers and other visual acuity measuring devices which include a pulsating light source. For example, in U.S. Pat. No. 4,021,102 issued to Iizuka on May 3, 1977, a refractometer is shown including a pair of infra-red light sources which are alternately flickered. The light beams generated by the light sources are passed through a vertical slit which is movable to permit focus of the slit image on the retina of the patient's eye. The slit image on the retina is reflected via a prism to a pair of photodetectors. The difference between the signals detected by the pair of photodetectors is then measured and, if no difference is detected, the visual acuity of the patient's eye is normal. However, if the eye is myopic or hyperopic, the slit image is not properly focused on the retina and the reflected slit image detected by the photodetectors results in a DC difference which actuates a compensating lens system to automatically correct the focus of the eye.
Finally, a number of other prior art patents show refractometers which use a primary target pattern for measuring the visual acuity of the patient's eyes. Some of these prior art patients use flashing or pulsating sources of light such as U.S. Pat. No. 3,843,240 issued to Cornsweet on Oct. 22, 1974. Others of these prior art refractometers and other visual acuity devices use a target pattern which is illuminated by a light source which is periodically interrupted by rotating drums or reticles. For example, the Munnerly patent cited above shows a rotating drum or reticle for interrupting the beam of light directed through the target pattern to the patient's eye.
Although all the above prior art devices objectively measure the acuity of a patient's eyes, and provide accurate and automatic arrangements for measuring visual acuity, these devices are not practical for mass vision testing because they require trained specialists for operation, they are large and complex mechanisms, and they are prohibitively expensive. Furthermore, these prior art devices generally are not flexible enough to measure both near and far visual acuity.